1. Field of the Invention
The present invention relates to a two-way optical communication device and a two-way optical communication system both capable of transmitting and receiving optical signals bidirectionally, and a method for assembling the two-way optical communication device. More particularly, the present invention relates to a two-way optical communication device and a two-way optical communication system both employed for home communication, device-to-device communication, a local area network (LAN), and the like, in which a multi-mode optical fiber, such as a plastic optical fiber, is used as a transmission path.
2. Description of the Related Art
As an information society develops, network technologies using fiber-optic communication have been a focus of attention. Particularly, such technologies are applied to household communication and device-to-device communication with the advent of recent plastic optical fibers having low light loss and broad band capabilities. Hereinafter, plastic optical fibers are also referred to as POFs.
Conventionally, a dominating optical communication system for transmitting and receiving optical signals having the same wavelength through an optical fiber(s) as a transmission medium is a system in which two optical fibers are used to perform full duplex optical communication. However, when two optical fibers are employed, it is difficult to reduce the size of an optical communication device, and the cost of the optical fibers is increased with an increase in a transmission distance. Therefore, a two-way optical communication device has been proposed in which a single optical fiber is used to perform full duplex optical communication.
In such a two-way optical communication device, transmission and reception are performed on the same optical fiber, so that it is important as to how to prevent interference between transmitted light and incoming light. Causes of incoming light interfering with transmitted light are: (1) when transmitted light enters an optical fiber, a portion of the light is reflected by an end surface of the optical fiber (hereinafter referred to as near-end reflection); (2) when transmitted light which has propagated through an optical fiber is emitted from the optical fiber, a portion of the light is reflected by an end surface of the optical fiber (hereinafter referred to as far-end reflection); (3) transmitted light is reflected by the two-way optical communication device on the other end (hereinafter referred to as reflection on the other-end module); (4) internally scattered light within the two-way optical communication device (hereinafter referred to as stray light); and the like. Further, there are problems other than the optical interference between transmitted light and incoming light, such as (5) electrical or electromagnetic noise. In this case, a signal-to-noise (S/N) ratio is reduced.
Japanese Laid-Open Publication No. 10-153720 discloses a representative method which has been conventionally proposed in order to solve the above-described problems. In this method, a polarization separation device (polarization separation film) is used to separate transmitted light from incoming light. This conventional technique will be described with reference to FIG. 16.
In a two-way optical communication device 1600 as shown in FIG. 16, transmitted light 108 emitted from a laser diode 104, which is in the form of S-polarization, enters a polarization reflection film 107 provided on a tilted surface of a prism 111. The transmitted light 108 is mostly reflected by the polarization reflection film 107, condensed by a lens 106, and coupled to an optical fiber 102. Incoming light 109 emitted from the multi-mode optical fiber 102 is condensed by the lens 106 and enters the polarization reflection film 107 in the form of random polarization. The substantial half of the incoming light 109 is reflected by the polarization reflection film 107 while the remaining half is transmitted by the polarization reflection film 107 to be coupled to a photodetector 105. In this case, the transmitted light 108 reflected by the optical fiber 102 is in the form of S-polarized light and therefore, is substantially perfectly reflected by the polarization reflection film 107 so as not to be coupled to the photodetector 105. Therefore, transmitted light of near-end reflection can be prevented from interfering with the incoming light.
Further, there is another known method which prevents transmitted light of near-end reflection from interfering with incoming light by providing a light blocking plate between a transmitter portion and a receiver portion. This conventional technique will be described with reference to FIG. 17.
In a two-way optical communication device 1700 as shown in FIG. 17, transmitted light 208 emitted from a light emitting element 204 is condensed by a transmission optical system 206 and coupled to an optical fiber 202. Incoming light 209 emitted from the optical fiber 202 is condensed by a reception optical system 224 and coupled to a photodetector 205. Further, a light blocking plate 207 made of metal or the like is provided between a transmitter portion and a receiver portion so that the transmitted light 208 reflected by the optical fiber 202 is prevented from being coupled to the photodetector 205.
Furthermore, Japanese Laid-Open Publication No. 62-222211 discloses a method in which transmitted light is condensed by a spheroid type mirror and coupled to an optical fiber. In this method, transmitted light emitted from a light emitting element is reflected from a concave mirror toward an optical fiber, condensed and coupled to an optical fiber. This concave mirror is in the shape of a spheroid. A light emitting element is provided at one of the two focus positions of the concave mirror, while an end surface of the optical fiber is provided at the other focus position. Therefore, transmitted light emitted from the light emitting element is conversed on the end surface of the optical fiber and coupled to the optical fiber. Similarly, if a photodetector is provided instead of the light emitting element, it is possible to efficiently receive incoming light emitted from an optical fiber.
In the method disclosed in the above-described Japanese Laid-Open Publication No. 10-153720, the substantial half of incoming light is reflected by the polarization reflection film 107, resulting in a reception loss of about 3 dB. Therefore, light cannot be efficiently used. In this conventional technique, transmitted light of near-end reflection can be prevented from interfering with incoming light. However, since light of far-end reflection and of reflection on an other-end module has random polarization directions, it is difficult to separate between transmitted light and incoming light. Further, since the conventional technique utilizes polarization, an inexpensive light emitting diode (LED) cannot be used as the light emitting element. Furthermore, in the conventional technique, an expensive polarization separation film (polarization reflection film) is required, leading to an increase in cost. Further still, in the conventional technique, the laser diode 104 is disposed close to the photodetector 105, both of which are provided on a substrate. Furthermore, since the laser diode 104 is not shielded, electrical or eletromagnetic noise easily occurs.
A problem with the two-way optical communication device 1700, in which the light blocking plate 207 is used to separate the transmitter portion and the receiver portion, is that the number of parts is increased raising the cost, and that a region of the optical fiber 202 corresponding to the thickness of the light blocking plate 207 cannot be used, resulting in a reduction in reception efficiency. Further, in this conventional technique, the degree of freedom in disposing the light emitting element 204 and the photodetector 205 is low. Therefore, the transmission optical system 206 and the reception optical system 224 need to be positioned with respect to each other with great precision, leading to an increase in manufacturing cost.
Further, the technique disclosed in Japanese Laid-Open Publication No. 62-222211 can be applied to two-way optical communication using two optical fibers, but not to two-way optical communication and full duplex communication which employ a single optical fiber.
According to one aspect of the present invention, a two-way optical communication device for use in a two-way optical communication system for transmitting and receiving an optical signal via a single optical fiber, comprises a light emitting element for generating transmitted light, a photodetector for receiving incoming light emitted from the optical fiber, and a reflection mirror made of a thin film having a high reflectance, having first and second surfaces, the first surface being opposite to the second surface. An optical member including the reflection mirror is provided closer to the optical fiber than the light emitting element. The incoming light emitted from the optical fiber is reflected by the first surface of the reflection mirror to be guided to the photodetector. The transmitted light emitted from the light emitting element or the transmitted light reflected by an end surface of the optical fiber is reflected by at least a portion of the second surface of the reflection mirror to prevent the transmitted light from entering the photodetector. Thereby, the above-described objective is achieved.
According to the above-described configuration, transmitted light reflected by the optical fiber and incoming light are separated by the reflection mirror which guides the incoming light to the photodetector, thereby making it possible to prevent interference between the transmitted light of near-end reflection and incoming light. Further, the reflection mirror provided in an optical member is used to separate the transmitted light and the incoming light, thereby reducing the number of parts as compared to conventional technologies using a polarization separation film or a light blocking plate to separate the transmitted light and the incoming light. Further, since light of near-end reflection can be separated by the reflection mirror, a reduction in efficiency in the use of light due to polarization separation losses as in a conventional technology using a polarization separation film does not occur. Since the separation of the light of near-end reflection is achieved by the reflection mirror made of a thin film, a reception region can be large so that a reduction in efficiency of the use of light due to a reduction in a reception region caused by the thickness of a light blocking plate in a conventional technology can be prevented. Furthermore, stray light which is scattered light occurring within the two-way optical communication device due to the reflection mirror can be prevented from entering the photodetector.
According to another aspect of the present invention, a two-way optical communication device for use in a two-way optical communication system for transmitting and receiving an optical signal via a single optical fiber, comprises a light emitting element for generating transmitted light, a photodetector for receiving incoming light emitted from the optical fiber, and a reflection mirror made of a thin film having a high reflectance, having first and second surfaces, the first surface being opposite to the second surface. An optical member including the reflection mirror is provided closer to the optical fiber than the light emitting element. The incoming light emitted from the optical fiber is reflected by the first surface of the reflection mirror to be guided to the photodetector. An optical absorbing layer is provided on at least a portion of the second surface of the reflection mirror, and the transmitted light emitted from the light emitting element or the transmitted light reflected by an end surface of the optical fiber is absorbed by the optical absorbing layer to prevent the transmitted light from entering the photodetector. Thereby, the above-described objective is achieved.
According to the above-described configuration, transmitted light reflected by the optical fiber and incoming light are separated by the reflection mirror which guides the incoming light to the photodetector, thereby making it possible to prevent interference between the transmitted light of near-end reflection and incoming light. Further, the reflection mirror provided in an optical member is used to separate the transmitted light and the incoming light, thereby reducing the number of parts as compared to conventional technologies using a polarization separation film or a light blocking plate to separate the transmitted light and the incoming light. Further, since light of near-end reflection can be separated by the reflection mirror, a reduction in efficiency in the use of light due to polarization separation losses as in a conventional technology using a polarization separation film does not occur. Since the separation of the light of near-end reflection is achieved by the reflection mirror made of a thin film, a reception region can be large so that a reduction in efficiency of the use of light due to a reduction in a reception region caused by the thickness of a light blocking plate in a conventional technology can be prevented. Furthermore, stray light which is scattered light occurring within the two-way optical communication device due to the reflection mirror can be prevented from entering the photodetector. Furthermore, stray light can be absorbed in the two-way optical communication device by the optical absorbing layer, thereby preventing interference.
In one embodiment of the present invention, the reflection mirror is in the shape of a curved surface, the incoming light emitted from the optical fiber is reflected and condensed by the reflection mirror, and the condensed incoming light is coupled to the photodetector.
According to the above-described configuration, since incoming light is condensed by a reflection mirror, an additional condensing lens is not required. Therefore, the number of parts is reduced, thereby making it easy to adjust assembly.
In one embodiment of the present invention, the transmitted light emitted from the light emitting element propagates through a portion of the optical member including the reflection mirror.
According to the above-described configuration, when a transmitter portion (through which transmitted light propagates) and a receiver portion (in which a photodetector is provided) are optically separated by the reflection mirror, the transmitted light is passed through a portion of the optical member in which the reflection mirror is provided-. Since the reflection mirror is made of a thin film, the transmission light can be passed in the vicinity of the receiver portion, thereby minimizing the boundary between a transmission region and a reception region (regions in the optical fiber) and therefore reducing the size of a two-way optical communication device.
In one embodiment of the present invention, the two-way optical communication device of the present invention further comprises a lens for condensing the transmitted light emitted from the light emitting element, wherein the lens is provided in the optical member.
According to the above-described configuration, a condensing optical system for transmission and reception can be constructed by a single optical member, resulting in a small-size, inexpensive, and easy-to-assemble two-way optical communication device.
In one embodiment of the present invention, the two-way optical communication device of the present invention further comprises a prism for refracting the transmitted light into the optical fiber, wherein the prism is provided in the optical member at a first position, the transmitted light being emitted from the first position to the optical fiber.
According to the above-described configuration, transmitted light is refracted by a prism so as to enter an optical fiber from a peripheral direction, thereby making it possible to enlarge the reception region of an optical fiber. Further, by using the prism, interference due to other-end module reflection can be suppressed. Furthermore, the degree of freedom of positioning of a light emitting element can be increased. The prism is integrated into the optical member, thereby obtaining a small-size and easy-to-assemble two-way optical communication device.
In one embodiment of the present invention, a surface of the optical member is used as a portion of a shielding member for shielding the light emitting element from the outside, the lens being provided on the surface of the optical member.
According to the above-described configuration, the optical member functions as a cover glass for the light emitting element, thereby reducing the number of parts and making it easy to assemble a two-way optical communication device.
In one embodiment of the present invention, an optical axis of the light emitting element is tilted with respect to an optical axis of the optical fiber.
According to the above-described configuration, a portion of incoming light emitted from the optical fiber is brought to and reflected by the light emitting element can be prevented from being coupled back to the optical fiber, resulting in a reduction in other-end module reflection.
In one embodiment of the present invention, a photodetecting surface of the photodetector is tilted with respect to an optical axis of the optical fiber.
According to the above-described configuration, a portion of incoming light emitted from the optical fiber is brought to and reflected by the photodetector can be prevented from being coupled back to the optical fiber, resulting in a reduction in other-end module reflection.
According to another aspect of the present invention, a two-way optical communication device for use in a two-way optical communication system for transmitting and receiving an optical signal via a single optical fiber, comprises a light emitting element for generating transmitted light, a photodetector for receiving incoming light emitted from the optical fiber, and a reflection mirror made of a thin film having a high reflectance, having first and second surfaces, the first surface being opposite to the second surface. An optical member including the reflection mirror is provided closer to the optical fiber than the photodetector. The incoming light emitted from the optical fiber is reflected by the first surface of the reflection mirror to be guided to the photodetector. The transmitted light reflected by an end surface of the optical fiber is reflected by the first surface of the reflection mirror to prevent the transmitted light from entering the photodetector. Thereby, the above-described objective is achieved.
According to the above-described configuration, transmitted light reflected by the optical fiber and incoming light are separated by the reflection mirror which guides the incoming light to the optical fiber, thereby making it possible to prevent interference between the transmitted light of near-end reflection and incoming light. Further, the reflection mirror provided in an optical member is used to separate the transmitted light and the incoming light, thereby reducing the number of parts as compared to conventional technologies using a polarization separation film or a light blocking plate to separate the transmitted light and the incoming light. Further, since light of near-end reflection can be separated by the reflection mirror, a reduction in efficiency in the use of light due to polarization separation losses as in a conventional technology using a polarization separation film does not occur. Since the separation of the light of near-end reflection is achieved by the reflection mirror made of a thin film, a reception region can be large so that a reduction in efficiency of the use of light due to a reduction in a reception region caused by the thickness of a light blocking plate in a conventional technology can be prevented. Furthermore, stray light which is scattered light occurring within the two-way optical communication device due to the reflection mirror can be prevented from entering the photodetector.
In one embodiment of the present invention, the reflection mirror is in the shape of a curved surface, the incoming light emitted from the optical fiber is reflected and condensed by the reflection mirror, and the condensed incoming light is coupled to the photodetector.
According to the above-described configuration, since incoming light is condensed by a reflection mirror, an additional condensing lens is not required. Therefore, the number of parts is reduced, thereby making it easy to adjust assembly.
In one embodiment of the present invention, the transmitted light emitted from the light emitting element propagates through a portion of the optical member including the reflection mirror.
According to the above-described configuration, when a transmitter portion (through which transmitted light propagates) and a receiver portion (in which a photodetector is provided) are optically separated by the reflection mirror, the transmitted light is passed through a portion of the optical member in which the reflection mirror is provided. Since the reflection mirror is made of a thin film, the transmission light can be passed in the vicinity of the receiver portion, thereby minimizing the boundary between a transmission region and a reception region (regions in the optical fiber) and therefore reducing the size of a two-way optical communication device.
In one embodiment of the present invention, the two-way optical communication device of the present invention further comprises a lens for condensing the incoming light to the photodetector, wherein the lens is provided in the optical member.
According to the above-described configuration, a condensing optical system for transmission and reception can be constructed by a single optical member, resulting in a small-size, inexpensive, and easy-to-assemble two-way optical communication device.
In one embodiment of the present invention, an optical axis of the light emitting element is tilted with respect to a direction perpendicular to an optical axis of the optical fiber.
According to the above-described configuration, a portion of incoming light emitted from the optical fiber is brought to and reflected by the light emitting element can be prevented from being coupled back to the optical fiber, resulting in a reduction in other-end module reflection.
In one embodiment of the present invention, a photodetecting surface of the photodetector is tilted with respect to a direction perpendicular to an optical axis of the optical fiber.
According to the above-described configuration, a portion of incoming light emitted from the optical fiber is brought to and reflected by the photodetector can be prevented from being coupled back to the optical fiber, resulting in a reduction in other-end module reflection.
In one embodiment of the present invention, a portion of the reflection mirror contacts or is close to an end surface of the optical fiber.
According to the above-described configuration, a portion of the reflection mirror (not the entire rear surface of the reflection mirror, but a light blocking portion provided at a tip portion of the rear surface of the reflection mirror at the optical fiber side) can reliably prevent interference between transmitted light of near-end reflection and incoming light.
In one embodiment of the present invention, the reflection mirror is electrically connected to a ground electrode of the photodetector.
According to the above-described configuration, the photodetector is electrically and electromagnetically separated from a light emitting element, thereby reducing electric and electromagnetic interference.
In one embodiment of the present invention, a photodetector positioning portion for setting a relative position of the photodetector with respect to the optical member is provided at a portion of the optical member, and the photodetector is positioned using the photodetector positioning portion.
According to the above-described configuration, the photodetector and the optical member can be directly positioned with respect to each other, thereby making it possible to position the photodetector and the optical member with high precision using an easy method.
In one embodiment of the present invention, a light emitting element positioning portion for setting a relative position of the light emitting element with respect to the optical member is provided at a portion of the optical member, and the light emitting element is positioned using the light emitting element positioning portion.
According to the above-described configuration, the light emitting element and the optical member can be directly positioned with respect to each other, thereby making it possible to position the light emitting element and the optical member with high precision using an easy method.
According to another aspect of the present invention, a two-way optical communication system comprises a plurality of two-way optical communication devices joined to respective ends of optical fibers. An optical signal is transmitted and received between at least two of the plurality of two-way optical communication device. At least one of the plurality of two-way optical communication devices is the two-way optical communication device of the present invention. Thereby, the above-described objective is achieved.
According to the above-described configuration, by using the two-way optical communication devices of the present invention capable of preventing interference between transmitted light and incoming light, full duplex communication can be realized using a single optical fiber, thereby obtaining a small-size and low-cost two-way optical communication system.
In one embodiment of the present invention, the end surface of the optical fiber is tilted with respect to an optical axis of the optical fiber.
According to the above-described configuration, light reflected when the light which has propagated through the optical fiber is emitted from the optical fiber (light of far-end reflection) can be prevented from entering the photodetector. Light reflected by an end surface of the optical fiber when entering the optical fiber is reflected by the optical fiber toward a peripheral direction of the optical fiber, thereby easily preventing interference between transmitted light of near-end reflection and incoming light. Furthermore, incoming light is refracted toward a photodetector side, thereby making it possible to couple incoming light to the photodetector with high efficiency.
In one embodiment of the present invention, the end surface of the optical fiber is in the shape of a spherical surface.
According to the above-described configuration, light reflected when the light which has propagated through the optical fiber is emitted from the optical fiber (light of far-end reflection) can be prevented from entering the photodetector. Light reflected by an end surface of the optical fiber when entering the optical fiber is reflected by the optical fiber toward a peripheral direction of the optical fiber, thereby easily preventing interference due to near-end reflection. Further, incoming light can be condensed and emitted from an end surface of the optical fiber, thereby making it possible to couple incoming light to the photodetector with higher efficiency. Furthermore, it is not necessary to restrict a specific connection direction between the optical fiber and the two-way optical communication device, resulting in easy connection.
In one embodiment of the present invention, the optical fiber is a plastic optical fiber including a core and a clad both made of plastic material.
According to the above-described configuration, the optical fiber is a POF, whereby a bend loss is small and the optical fiber is unlikely to be broken. Further, an optical fiber having a large core diameter of about 1 mm can be easily produced, thereby making it possible to easily join the optical fiber to a two-way optical communication device (positioning), and to produce an inexpensive two-way optical communication system.
In one embodiment of the present invention, the optical fiber is a polymer clad fiber including a clad made of plastic material and a core made of quartz.
According to the above-described configuration, the optical fiber is a PCF, whereby a transmission band is broad and long-distance and high-speed communication can be realized.
According to another aspect of the present invention, a method is provided for assembling the two-way optical communication device of the present invention. A receiver portion assembling member for setting relative positions of the portion of the optical member and the photodetector positioning portion by contacting the receiver portion assembling member with the portion of the optical member and the photodetector positioning portion is used to position the photodetector and the optical member. Thereby, the above-described objective is achieved.
According to the above-described method, the light emitting element positioning portion provided in the optical member including the reflection mirror, the condensing lens, and the like and the photodetector are contacted with the receiver portion assembling member. Therefore, by using such a simple method, the photodetector and the optical member can be positioned with respect to each other, thereby making it possible to assemble a two-way optical communication device at low cost.
According to another aspect of the present invention, a method is provided for assembling the two-way optical communication device of the present invention. The light emitting element positioning portion of the optical member and a portion of the light emitting element are positioned with respect each other, or the light emitting element positioning portion of the optical member and a holding portion having the light emitting element attached thereto are positioned with respect each other so as to position the light emitting element and the optical member. Thereby, the above-described objective is achieved.
According to the above-described method, by using the light emitting element positioning portion provided in the optical member including the reflection mirror, the condensing lens, and the like, the photodetector or the holding portion having the light emitting element (submount) and the optical member can be positioned with respect to each other with high precision.
Thus, the invention described herein makes possible the advantages of providing (1) an inexpensive and small-size two-way optical communication device and a two-way optical communication system capable of performing full duplex two-way optical communication using a single optical fiber, in which losses of transmitted light and incoming light are small, the interference between incoming light and transmitted light is suppressed and electrical or electromagnetic noise is suppressed, and light can be coupled to an optical fiber having a large diameter, such as a POF; and (2) a method for assembling the two-way optical communication device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.